Abstract
During earthquake propagation, geologic faults lose their strength, then strengthen as slip slows and stops. Many slip-weakening mechanisms are active in the upper-mid crust, but healing is not always well-explained. Here we show that the distinct structure and rate-dependent properties of amorphous nanopowder (not silica gel) formed by grinding of quartz can cause extreme strength loss at high slip rates. We propose a weakening and related strengthening mechanism that may act throughout the quartz-bearing continental crust. The action of two slip rate-dependent mechanisms offers a plausible explanation for the observed weakening: thermally-enhanced plasticity, and particulate flow aided by hydrodynamic lubrication. Rapid cooling of the particles causes rapid strengthening, and inter-particle bonds form at longer timescales. The timescales of these two processes correspond to the timescales of post-seismic healing observed in earthquakes. In natural faults, this nanopowder crystallizes to quartz over 10s–100s years, leaving veins which may be indistinguishable from common quartz veins.
Highlights
IntroductionGeologic faults lose their strength, strengthen as slip slows and stops
During earthquake propagation, geologic faults lose their strength, strengthen as slip slows and stops
We have shown that the lubricating wear material produced in moderate to fast slip rate experiments on novaculite consists of spherical nanoparticles of amorphous silica with hydrated surfaces
Summary
Geologic faults lose their strength, strengthen as slip slows and stops. We show that the distinct structure and ratedependent properties of amorphous nanopowder (not silica gel) formed by grinding of quartz can cause extreme strength loss at high slip rates. The action of two slip rate-dependent mechanisms offers a plausible explanation for the observed weakening: thermally-enhanced plasticity, and particulate flow aided by hydrodynamic lubrication. Rapid cooling of the particles causes rapid strengthening, and inter-particle bonds form at longer timescales The timescales of these two processes correspond to the timescales of post-seismic healing observed in earthquakes. The proposal of “silica gel” lubrication, thought to cause extreme loss of shear resistance in high velocity friction experiments on quartz-rich rock, offered a tantalizing explanation for dramatic coseismic weakening which might apply broadly within continental crust[5,6]. We show the composition and structure of the frictional wear material and propose an alternative explanation for the lubricating and healing behaviors
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